The present invention relates to novel solid oxide fuel cell units and a fuel cell stack formed from such units.
In a solid oxide fuel cell, oxidant and fuel are electrochemically reacted without burning to produce electricity directly. The reactants are supplied to the cell through manifolds and channels that direct reactants to the appropriate sides of a solid ceramic membrane that acts as an electrolyte.
A conventional planar fuel cell stack is made from a plurality of interleaved ceramic membranes and interconnect plates which act as barriers between the anode of one cell and the cathode of the adjacent cell. Each individual interconnect plate is sealed to adjacent interconnect plates, and in addition each fuel and oxidant manifold within the interconnect plate is individually sealed. The seals are necessary to prevent mixing of fuel and oxidant gases. If the integrity of the stack is not maintained, the seals may leak and allow fuel and oxidant to mix. Because the fuel cell typically operates above the autoignition temperatures of the fuel gases, a fuel leak may be disastrous. As a result, the fuel cell stack must be painstakingly assembled to ensure the integrity of all the seals between the interconnect plates and fuel cell elements.
The high operating temperature of solid oxide fuel cells in excess of 600xc2x0 C., limits the selection of materials available for use as an interconnect. These materials must be able to withstand this temperature, and to simultaneously withstand an oxidizing environment on one side of the interconnect, and a partial reducing environment on the other. The material is also required to simultaneously maintain good electrical conductivity to collect the current generated by the cells. Most prior art interconnects have used ceramic materials and composites, however these materials demonstrate inferior electrical conductivity as compared to metals, and typically are not successful in withstanding both oxidizing and reducing environments simultaneously. Ceramic materials also are expensive to purchase as raw materials, require moulding or other processing, and then firing or sintering. These steps are all labour intensive and require significant amounts of time to process. In addition, a solid oxide fuel cell stack requires fine tolerances which are difficult to maintain when a green ceramic is sintered. Further, ceramic materials are brittle, and there can be significant losses during production due to handling and processing damage that occurs in the manufacture of the interconnect. Ceramic materials are also vibration and shock intolerant, which makes them unsuitable for applications where these factors are present, such as in automobiles.
Metallic interconnects which are machined from solid metal plates are known but are difficult to manufacture and as a result are expensive. There have been attempts to form metallic interconnects by bonding stacked metal plates together however such attempts have not been successful because of leaks forming between the metal plates and their inability to withstand the operating temperatures of solid oxide fuel cells. For example, U.S. Pat. No. 3,484,298 discloses a laminated electrode backing plate which is laminated using adhesives or other bonding agents. Further, in the prior art, the cells are held in place next to metal interconnect plates by thin inorganic seals, often alumina felts. These felts are quite thin, and require the cells and interconnect to be held together under compression to form an effective seal to prevent the fuel and oxidant gasses from leaking out of the assembled fuel cell stack. It is difficult to apply a compressive force using a mechanism entirely inside the hot zone, due to creep of materials at such elevated temperatures. It is essential that this compression means apply uniform force to the stack, and not exceed a threshold value, otherwise the brittle ceramic cells will crack, resulting in a failed stack assembly.
Prior art fuel cell interconnects have had to be constructed with exotic materials and construction techniques to maintain the stacks seal integrity, and thus have been difficult and expensive to manufacture. Prior art interconnects have had very fine tolerances, and have required many labour intensive steps to be made into a fuel cell stack. This had prohibited their use in mass produced applications.
Accordingly, there is a need in the art for a method of sealing the ceramic cells and stacking them with interconnect, such that the difficulties of assembly and integrity of a seal can be mitigated, whilst providing a means of shock isolation, such that the difficulties of using a brittle cell element can be minimized.
The present invention relates to a novel fuel cell unit including a cassette holder for a ceramic fuel cell element sandwiched between interconnects of a novel deign and also relates to the fuel cell stack formed by the novel fuel cell units. The cassette holder isolates the cell from the surrounding interconnect by means of a pliant seal within a rigid frame, thereby reducing the possibility of breakage of the brittle cells.
Accordingly in one aspect of the invention, the invention comprises fuel cell unit apparatus comprising:
(a) an upper interconnect comprising a top plate and a lower plate enclosing a sealed interior chamber and defining an intake and an exhaust manifold opening in fluid communication with the chamber, wherein the lower plate defines a cell opening;
(b) a lower interconnect comprising a lower plate and an upper plate enclosing a sealed chamber defining an intake and an exhaust manifold opening in fluid communication with the chamber, wherein the upper plate defines a cell opening;
(c) a fuel cell cassette comprising:
(i) a single planar fuel cell element having an anode surface, a cathode surface and an edge surface;
(ii) a resilient seal element which contacts both flat surfaces and the edge surface; and
(iii) a frame that retains both the seal and the ceramic cell element; wherein the fuel cell cassette fits within the upper interconnect cell opening and mates with the upper interconnect to seal the upper chamber and the fuel cell cassette fits within the lower interconnect cell opening and mates with the lower interconnect to seal the lower chamber; and
(d) seal means disposed between the upper and lower interconnects.
In one embodiment, interconnects may preferably be substantially rhomboidal in shape with a square central portion and two outwardly projecting manifold portions on opposite sides of the central portion. In one embodiment, the cassette frame is comprised of an upper and a lower portion that are joined to retain the seal in place between the two portions. In one embodiment the seal is made from a flexible resilient material and in another the seal comprises a matrix of ceramic fibres. It is anticipated that the fibres could comprise alumina, zirconia or combinations of both, In a further embodiments, the seal further comprises a ceramic powder that could be zirconia powder or alumina powder or a combination of both.
In another aspect of the invention, the invention comprises a fuel cell stack which is formed from a stacked plurality of the fuel cell units described above.